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You don't need to perform any join at all.

SELECT ParentId AS [Post Link]
     , OwnerUserId AS [User Link]
     , COUNT(Id) AS Answers
    FROM Posts
    WHERE
        ParentId IS NOT NULL           -- ← Answers only
        AND OwnerUserId <> 0
    GROUP BY ParentId, OwnerUserId
    HAVING Count(id) > 1
    ORDER BY Answers DESC;

Your instinct probably tells you that you need a JOIN to obtain the question titles. However, in Stack Exchange Data Explorer, there is a special feature: provide a PostId in a column named [Post Link], and it will take care of the presentation for you (implicitly doing a JOIN).